The intent of this project is to vary one or more at a time of the variables relevant to muscle oxygen use such as blood flow, blood PO2, blood glucose concentration, contraction freqeuency, contraction load, etc., in order to discover and quantitate which of the many such variables are causually related to concurrent changes in muscle oxygen uptake. The studies will be done on in situ dog muscles. Oxygen uptake will be calculated from measurements of blood flow and arterial and venous blood oxygen contents. The variables currently under investigation are muscle load and length during isotonic twitch and tetanic contractions and muscle blood flow and blood oxygen content during steady state isometric twitch contractions.